An internal combustion engine is typically equipped with a lubrication system that comprises such components as an oil pump, oil filter, oil strainer and relief valve for feeding pressurizing engine oil to various parts of the engine to lubricate the valve mechanism and cool the pistons. There are various types of oil filters, and a spin-on type oil filter is used in a majority of small to medium sized engines commonly used for passenger vehicles. A spin-on type oil filter comprises a filter element typically formed by folding filtering paper and an enclosed casing, and is typically screwed into a threaded boss projecting typically from a side of a crankcase.
Conventionally, most spin-on type oil filters have been offered as disposable type oil filters. However, when removed from an engine at the end of a service life, a disposable oil filter cannot be easily separated into a filter element and a casing for garbage recycling, and it is desirable to reuse the casing for the preservation of natural resources. Therefore, proposals have been made to use reusable oil filters using a replaceable filter element instead of disposable oil filters. In a reusable oil filter, it is necessary that a casing accommodating a filter element and relief valve is detachably fastened to a set plate provided with a female thread for mounting on an engine and defining oil passages for the engine oil.
Reusable oil filters were commonly used before the disposable oil filters gained popularity, but were taken over by disposable oil filters because replacing filter elements is a very messy process that could result in a good deal of oil spill. A known method for fastening a casing of a reusable oil filter to a set plate uses a thread structure that includes a male thread formed around the set plate and a ring nut fitted on the casing and provided with a female thread that can be threaded with the male thread of the set plate (see Japanese patent No. 3076495 and Japanese patent laid-open publication No. 2002-122014, for instance)
Such a thread structure requires the ring nut to be turned several times by using a fastening tool (such as a large-diameter ring wrench) when assembling the oil filter. Therefore, a considerable time is required for replacing oil filter elements. Also, the fastening torque must be properly controlled for the oil filter to be assembled in a reliable manner, and this adds to the work hours that is required for the assembly work. To ensure an adequate fastening force to secure the set plate, the male thread formed around the set plate is required to have a certain length, and this increases the size and weight of the container having a detachable lid.